Waveguide device incorporating a light pipe

ABSTRACT

A waveguide apparatus has in combination: a light pipe with an optical axis for guiding light therethrough; a light coupling element in optical contact with an elongate portion of the reflecting surface of the light guide; and an optical waveguide in optical contact with the coupling element.

PRIORITY CLAIMS

This application claims priority from U.S. Provisional Application Ser.No. 62/177,494 entitled WAVEGUIDE DEVICE INCORPORATING A LIGHT PIPEfiled on 16 Mar. 2015, which is hereby incorporated by reference in itsentirety.

CROSS REFERENCE TO RELATED APPLICATIONS

The following patent applications are incorporated by reference hereinin their entireties: U.S. patent application Ser. No. 13/506,389entitled COMPACT EDGE ILLUMINATED DIFFRACTIVE DISPLAY, U.S. Pat. No.8,233,204 entitled OPTICAL DISPLAYS, PCT Application No.: US2006/043938,entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY, PCTApplication No.: GB2012/000677 entitled WEARABLE DATA DISPLAY, U.S.patent application Ser. No. 13/317,468 entitled COMPACT EDGE ILLUMINATEDEYEGLASS DISPLAY, U.S. patent application Ser. No. 13/869,866 entitledHOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patent application Ser. No.13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY, U.S. Pat. No.8,224,133 entitled LASER ILLUMINATION DEVICE, U.S. Pat. No. 8,565,560entitled LASER ILLUMINATION DEVICE, U.S. Pat. No. 6,115,152 entitledHOLOGRAPHIC ILLUMINATION SYSTEM, PCT Application No.: PCT/GB2013/000005entitled CONTACT IMAGE SENSOR USING SWITCHABLE BRAGG GRATINGS, PCTApplication No.: PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHICPOLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES, PCT ApplicationNo.: PCT/GB2014/000197 entitled HOLOGRAPHIC WAVEGUIDE EYE TRACKER, U.S.Provisional Patent Application No. 62/071,534 entitled HOLOGRAPHICWAVEGUIDE FOR TRACKING AN OBJECT IN 3D SPACE, PCT/GB2013/000210 entitledAPPARATUS FOR EYE TRACKING, PCT Application No.: GB2013/000210 entitledAPPARATUS FOR EYE TRACKING.

BACKGROUND OF THE INVENTION

This invention relates to a waveguide device, and more particularly to awaveguide holographic grating device incorporating a light pipe.Waveguide optics is currently being considered for a range of displayand sensor applications for which the ability of waveguides to integratemultiple optical functions into a thin, transparent, lightweightsubstrate is of key importance. This new approach is stimulating newproduct developments including near-eye displays for Augmented Reality(AR) and Virtual Reality (VR), compact Heads Up Display (HUDs) foraviation and road transport and sensors for biometric and laser radar(LIDAR) applications. A common requirement in waveguide optics is toprovide beam expansion in two orthogonal directions. In displayapplications this translates to a large eyebox. While the principles ofbeam expansion in holographic waveguides are well established dual axisexpansion requires separate grating layers to provide separate verticaland horizontal expansion. One of the gratings usually the one giving thesecond axis expansion also provides the near eye component of thedisplay where the high transparency and thin, lightweight form factor ofa diffractive optics can be used to maximum effect. In practical displayapplications, which demand full color and large fields of view thenumber of layers required to implement dual axis expansion becomesunacceptably large resulting in increased thickness weight and haze.Solutions for reducing the number of layers based on multiplexing two ormore gratings in a single layer or fold gratings which can perform dualaxis expansion (for a given angular range and wavelength) in a singlelayer are currently in development. Dual axis expansion is also an issuein waveguides for sensor applications such as eye trackers and LIDAR.There is a requirement for a low cost, efficient means of generating thefirst axis expansion in a dual axis expansion waveguide.

SUMMARY OF THE INVENTION

It is a first object of the invention to provide a low cost, efficientmeans of generating the first axis expansion in a dual axis expansionwaveguide.

The object of the invention is achieved in first embodiment of theinvention in which there is provided a waveguide apparatus comprising incombination: a light pipe with an optical axis for guiding lighttherethrough; a light coupling element in optical contact with anelongate portion of the reflecting surface of the light guide; and anoptical waveguide in optical contact with the light coupling element.

In one embodiment the light is transmitted from the light pipe to theoptical waveguide via the light coupling element.

In one embodiment the light is transmitted from the optical waveguide tothe light pipe via the light coupling element.

In one embodiment the reflecting surface comprises abutting mutuallyinclined elongate elements. At least one elongate element is in opticalcontact with the light coupling element.

In one embodiment the reflecting surface comprises abutting elongateelements mutually inclined at a common angle. At least one of theelongate elements is in optical contact with the light coupling element.

In one embodiment the coupling element is one of a grating, a beamsplitter, an evanescent coupling optical medium, or a gradient indexoptical medium.

In one embodiment the light coupling element is a fold grating.

In one embodiment the light coupling element is a surface reliefgrating.

In one embodiment the light coupling element couples light characterizedby at least one of angular range, wavelength range or polarizationstate.

In one embodiment the light coupling element is a Bragg grating, aswitchable Bragg grating or an array of selectively switchable elements.The coupling element is recorded in one of a HPDLC grating, uniformmodulation grating or reverse mode HPDLC grating.

In one embodiment the light coupling element is a grating comprising atleast two multiplexed gratings.

In one embodiment the light coupling element is a grating having atleast one of the characteristics of spatially varying thickness,spatially-varying diffraction efficiency, or spatially-varying k-vectordirections.

In one embodiment the optical medium of the light pipe is at least oneof air, optical refractive material or a gradient index material.

In one embodiment the optical waveguide contains a grating operative toextract light propagating therethrough out of the optical waveguide or agrating operative to couple-in light from outside the optical waveguide.

In one embodiment the light guide device further comprises a couplinggrating in optical contact with the light pipe. The coupling grating hasa non zero clock angle with respect to the optical axis.

In one embodiment the light guide device further comprises a light pipecoupling grating in optical contact with the light pipe. The opticalwaveguide contains a grating having a reciprocal diffractiverelationship with the light pipe coupling grating.

In one embodiment the light guide device further comprises a light pipecoupling grating in optical contact with the light pipe. The the lightpipe coupling grating couples light modulated with temporally-varyingangularly-distributed information content into the light pipe.

In one embodiment the light guide device further comprises a light pipecoupling grating in optical contact with the light pipe. The couplinggrating couples data modulated light out of the light pipe.

In one embodiment the light pipe is divided into two elongate portionsby a beamsplitter layer.

In one embodiment the light pipe is curved.

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein like index numerals indicate like parts.For purposes of clarity, details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic three dimensional illustration of a waveguideapparatus in one embodiment.

FIG. 2A is a cross section view of a light pipe of a light pipe with asquare cross-section.

FIG. 2B is a cross section view of a light pipe of a light pipe with atriangular cross-section.

FIG. 2C is a cross section view of a light pipe of a light pipe with ahexagonal cross-section.

FIG. 2D is a cross section view of a light pipe of a light pipe with around cross-section.

FIG. 3A is a plan view of a waveguide apparatus in one embodiment. FIG.3B is a side elevation view of the embodiment of FIG. 3A.

FIG. 3C is a plan view of a basic fold grating.

FIG. 4 is a schematic three dimensional illustration of a waveguideapparatus using a fold grating light coupling element in one embodiment.

FIG. 5 is a detail of a waveguide apparatus using a beamsplitter lightcoupling element in one embodiment.

FIG. 6 is a plan view of a waveguide apparatus using a clocked couplinggrating in one embodiment.

FIG. 7A is a side elevation view of a hollow cavity square cross-sectionlight pipe in one embodiment.

FIG. 7B is a cross section view of a hollow cavity square cross-sectionlight pipe in one embodiment.

FIG. 8A is a side elevation view of a gradient index squarecross-section light pipe in one embodiment.

FIG. 8B is a plot of the lateral refractive index distribution of agradient index square cross-section light pipe in one embodiment.

FIG. 9A is a side elevation view of a light pipe square cross-sectionincluding a beamsplitter layer in one embodiment.

FIG. 9B is a cross section view of a light pipe square cross-sectionincluding a beamsplitter layer in one embodiment.

FIG. 10 is a side elevation view of a light pipe light coupling elementbased on a grating in one embodiment.

FIG. 11 is a side elevation view of a light pipe light coupling elementbased on a beamsplitter in one embodiment.

FIG. 12 is a side elevation view of a light pipe light coupling elementbased on a surface relief grating in one embodiment.

FIG. 13 is a side elevation view of a light pipe light coupling elementbased on a switchable grating array in one embodiment.

FIG. 14 is a side elevation view of a light pipe light coupling elementbased on a gradient index layer in one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be further described by way of example only withreference to the accompanying drawings. It will apparent to thoseskilled in the art that the present invention may be practiced with someor all of the present invention as disclosed in the followingdescription. For the purposes of explaining the invention well-knownfeatures of optical technology known to those skilled in the art ofoptical design and visual displays have been omitted or simplified inorder not to obscure the basic principles of the invention. Unlessotherwise stated the term “on-axis” in relation to a ray or a beamdirection refers to propagation parallel to an axis normal to thesurfaces of the optical components described in relation to theinvention. In the following description the terms light, ray, beam anddirection may be used interchangeably and in association with each otherto indicate the direction of propagation of light energy alongrectilinear trajectories. Parts of the following description will bepresented using terminology commonly employed by those skilled in theart of optical design. It should also be noted that in the followingdescription of the invention repeated usage of the phrase “in oneembodiment” does not necessarily refer to the same embodiment.

The grating used in the invention is desirably a Bragg grating (alsoreferred to as a volume grating). Bragg gratings have high efficiencywith little light being diffracted into higher orders. The relativeamount of light in the diffracted and zero order can be varied bycontrolling ther refractive index modulation of the grating, a propertywhich is used to make lossy waveguide gratings for extracting light overa large pupil. One important class of gratings is known as SwitchableBragg Gratings (SBG). SBGs are fabricated by first placing a thin filmof a mixture of photopolymerizable monomers and liquid crystal materialbetween parallel glass plates. One or both glass plates supportelectrodes, typically transparent indium tin oxide films, for applyingan electric field across the film. A volume phase grating is thenrecorded by illuminating the liquid material (often referred to as thesyrup) with two mutually coherent laser beams, which interfere to form aslanted fringe grating structure. During the recording process, themonomers polymerize and the mixture undergoes a phase separation,creating regions densely populated by liquid crystal micro-droplets,interspersed with regions of clear polymer. The alternating liquidcrystal-rich and liquid crystal-depleted regions form the fringe planesof the grating. The resulting volume phase grating can exhibit very highdiffraction efficiency, which may be controlled by the magnitude of theelectric field applied across the film. When an electric field isapplied to the grating via transparent electrodes, the naturalorientation of the LC droplets is changed causing the refractive indexmodulation of the fringes to reduce and the hologram diffractionefficiency to drop to very low levels. Typically, SBG Elements areswitched clear in 30 μs. With a longer relaxation time to switch ON.Note that the diffraction efficiency of the device can be adjusted, bymeans of the applied voltage, over a continuous range. The deviceexhibits near 100% efficiency with no voltage applied and essentiallyzero efficiency with a sufficiently high voltage applied. In certaintypes of HPDLC devices magnetic fields may be used to control the LCorientation. In certain types of HPDLC phase separation of the LCmaterial from the polymer may be accomplished to such a degree that nodiscernible droplet structure results. A SBG may also be used as apassive grating. In this mode its chief benefit is a uniquely highrefractive index modulation.

SBGs may be used to provide transmission or reflection gratings for freespace applications. SBGs may be implemented as waveguide devices inwhich the HPDLC forms either the waveguide core or an evanescentlycoupled layer in proximity to the waveguide. The parallel glass platesused to form the HPDLC cell provide a total internal reflection (TIR)light guiding structure. Light is coupled out of the SBG when theswitchable grating diffracts the light at an angle beyond the TIRcondition. Waveguides are currently of interest in a range of displayand sensor applications. Although much of the earlier work on HPDLC hasbeen directed at reflection holograms transmission devices are provingto be much more versatile as optical system building blocks. Typically,the HPDLC used in SBGs comprise liquid crystal (LC), monomers,photoinitiator dyes, and coinitiators. The mixture frequently includes asurfactant. The patent and scientific literature contains many examplesof material systems and processes that may be used to fabricate SBGs.Two fundamental patents are: U.S. Pat. No. 5,942,157 by Sutherland, andU.S. Pat. No. 5,751,452 by Tanaka et al. Both filings describe monomerand liquid crystal material combinations suitable for fabricating SBGdevices. One of the known attributes of transmission SBGs is that the LCmolecules tend to align normal to the grating fringe planes. The effectof the LC molecule alignment is that transmission SBGs efficientlydiffract P polarized light (ie light with the polarization vector in theplane of incidence) but have nearly zero diffraction efficiency for Spolarized light (ie light with the polarization vector normal to theplane of incidence. Transmission SBGs may not be used at near-grazingincidence as the diffraction efficiency of any grating for Ppolarization falls to zero when the included angle between the incidentand reflected light is small.

The object of the invention is achieved in first embodiment illustratedin FIG. 1 in which there is provided a waveguide apparatus comprising alight pipe 100 with an optical axis 1000A; a light coupling element 101in optical contact with an elongate portion of the reflecting surface ofthe light guide; and an optical waveguide 102 in optical contact withthe coupling element. In most embodiments the optical axis is an axis ofsymmetry (corresponding to the intersections of the normals to thereflecting surfaces the light pipe). The light pipe is square in crosssection and has elongate reflecting surfaces. 100L, 100R, 100T, 100Bwhere characters L,R,T,B refer to the left, right, top and bottomsurfaces of the light pipe respectively One TIR ray path in the lightpipes is represented by the rays 1000-1008. The ray follows a cyclic orspiral path down the light pipe. The means for producing the spiral pathwill be discussed later. The interactions of the rays with the lightpipe surface are indicated by the points labelled 1001L 1001T,1001R,1001B, to 1002L 1002T,1002R, 1002B, 1003L. The surface 100L is inoptical contact with the light coupling element such that rays strikingthe points 1001L,1002,1003L are coupled into the optical waveguide asindicated by the rays 1009-1011. Note that in FIG. 1 light is shownbeing transmitted from the light pipe to the optical waveguide via thelight coupling element. In other embodiments the light is transmittedfrom the optical waveguide to the light pipe via the light couplingelement. Such embodiments are described by FIG. 1 with the ray arrowsare reversed. In one embodiment the light pipe is curved.

For simplicity of explanation we shall continue to consider light pipesof square cross section. However, the invention may be used with lightpipes of more generalized cross section such as the examples shown inFIG. 2. In one group of embodiments the reflecting surface comprisesabutting mutually inclined elongate planar elements with at least oneelongate element in optical contact with the coupling element. FIG. 2Ashows s typical spiral ray path projection 1013 for the embodiment ofFIG. 1. The light pipes may also have regular polygonal cross sectionssuch as the triangular one shown in FIG. 2B and the hexagonal crosssection shown in FIG. 2C. In one embodiment the cross section may becircular as sown in FIG. 2D. In other embodiments the light pipe mayhave a reflective surface built up from elongate elements of higherorder curvatures. In most practical application it will be advantagesfor the elongated surface elements to be equidistant from the opticalaxis of the light pipe. In other words the abutting elongate elementswould be mutually inclined at a common angle with at least one theelongate element in optical contact with the coupling element.

The coupling element may be based on a grating or beam splitter. In oneembodiment the coupling element may be a gradient index optical medium.In another embodiment the coupling element may be an evanescent couplingoptical medium. There are several options for implement a couplingelement based on a grating. In one embodiment the coupling element is asurface relief grating. A Bragg grating offers greater angle andwavelength selectivity. In one embodiment a switchable Bragg gratingrecorded in HPDLC, as discussed above may be used. The same technologymay be used to provide an array of selectively switchable elementsdisposed along the length of the light pipe. In one embodiment thecoupling element is based on a grating recorded in a uniform modulationgrating. Exemplary uniform modulation liquid crystal-polymer materialsystems are disclosed in United State Patent Application PublicationNo.: US2007/0019152 by Caputo et al and PCT Application No.:PCT/EP2005/006950 by Stumpe et al. both of which are incorporated hereinby reference in their entireties. Uniform modulation gratings arecharacterized by high refractive index modulation (and hence highdiffraction efficiency) and low scatter. In one embodiment the couplingelement is based on a grating recorded in a a reverse mode HPDLCmaterial. Reverse mode HPDLC differs from conventional HPDLC in that thegrating is passive when no electric field is applied and becomesdiffractive in the presence of an electric field. The reverse mode HPDLCmay be based on any of the recipes and processes disclosed in PCTApplication No.: PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHICPOLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES. The grating maybe recorded in any of the of the above material systems but used in apassive (non-switching) mode. The fabrication process is identical tothat used for switched but with the electrode coating stage beingomitted. LC polymer material systems are highly desirable in view oftheir high index modulation.

In one embodiment shown in FIG. 3 the waveguide apparatus comprises alight pipe 120, a light coupling element 121 comprising a fold gratingand an optical waveguide 122. The components are shown in plan view inFIG. 3A and in cross section in FIG. 3B. Input light 1020 is coupledinto the light pipe by the grating 123 and follows a spiral pathindicated in projection by the rays 1021-1022 in FIG. 3A and 1030 inFIG. 3B. Rays intersecting the light pipe face that is in opticalcontact with the optical coupling element, for example the rays atinteraction points 1023A, 1023B, are coupled in the waveguide device.The coupling light paths are illustrated schematically by the rays1023A,1023B in FIG. 3A and 1031 in FIG. 3B. The waveguide devicecontains an extraction grating 124 which diffracts a portion of theguided beam out of the waveguide at each beam-grating interaction alongthe TIR path 1032 (FIG. 3B). The paths in the waveguide device are alsoindicated by the rays 1028,1029 in FIG. 3A. The light extracted from theoptical waveguide is indicated by 1033. In a display application thislight would be directed to the eyebox. The coupling element is a foldgrating. This type of grating is normally used for changing beamdirection and providing beam expansion within a waveguide. Gratingsdesigned for coupling light into or out of a waveguides are tiltedaround an axis lying in the waveguide plane. Fold gratings have a moregeneralized tilt. In their simplest implementation, as used in thepresent invention, they are tilted around an axis perpendicular to thewaveguide plane such they deflect beams in the waveguide plane. Moregenerally, they may have tilts defined by two rotation angles so that,for example, light can be coupled into the waveguide and deflected intoan orthogonal direction inside the waveguide, all in one step. FIG. 3Cis a plan view of a basic fold grating 126. When the set of rays 1035encounter the grating, they diffract in a manner that changes thedirection of propagation by 90°. Unlike a conventional verticalextraction grating, the light does not leave the waveguide. Note thatwhen a ray encounters the grating, regardless of whether it intersectsthe grating from above or below, a fraction of it changes direction andthe remainder continues unimpeded. A typical ray will interact manytimes with vertically (in the Y direction) while some light will bemoving laterally (in the X direction). From a design perspective, it isdesirable to engineer the amount of light 1036 emerging from the outputedge of the grating to be uniformly distributed laterally and the amountof light 1037 emerging from the side edge of the grating to be as smallas possible.

In one embodiment the coupling element couples light characterized by atleast one of angular range, wavelength range or polarization state.Bragg transmission gratings are characterized by a high degree ofangular selectivity. The coupling element may also be based on abeamsplitter design to operate over a defined angular or wavelengthrange. In one embodiment the coupling elements is based on abirefringent grating. The index of such gratings has two components:extraordinary (n_(e)) and ordinary (n_(o)) indices. The extraordinaryindex is defined by the optic axis (ie axis of symmetry) of a uniaxialcrystal as determined by the average LC director direction. The ordinaryindex corresponds to the other two orthogonal axes. More generally theindex is characterised using a permittivity tensor. To the best of theinventors' knowledge the optic axis in LC-based gratings tends to alignnormal to the Bragg fringes ie along the K-vectors. For reasonably smallgrating slant angles applying an electric field across the cellre-orients the directors normal to the waveguide faces, effectivelyclearing the grating. An incident ray sees an effective index dependenton both the extraordinary and ordinary indices with the result that thePoynting vector and wave vector are separated by a small angle. Thiseffect becomes more pronounced at higher angles. In one embodiment thediffracted rays have a polarization state produced by aligning theaverage relative permittivity tensor of the grating. It is also usefulin some applications to have the capability of controlling thepolarization of non-diffracted light. Accordingly, in one embodiment thenon-diffracted rays have a polarization state produced by aligning theaverage relative permittivity tensor of the grating. The polarizationstates may be one of randomly, linearly or elliptically polarized. Inapplications where the diffracted light interacts with another gratingis desirable that it is linearly polarized. For example, SBGs havehighest diffraction efficiency for P-polarized light. In a waveguide thebirefringence of the LC will tend to rotate the polarization of thelight at each TIR bounce. This has the effect of scrambling thepolarization of the light. Initial experiments point to the light notbecoming fully randomly polarized. However, this is likely to depend onthe characteristics of the birefringence. In one embodiment thepermittivity tensor is modified to provide a random polarization stateat the output end of the grating. Random polarization is desirable inapplications in which the diffracted light is viewed directly, forexample in a display.

In one embodiment shown in FIG. 4 the waveguide apparatus comprises thelight pipe 131, which has a square cross section 132, a coupling grating133, a light coupling element 134 and an optical waveguide 135. A prismmay be used as an alternative to the coupling grating in someembodiments. Input light 1040 is coupled into the light pipe by thecoupling grating and follows a spiral path indicated by the rays1041-1042. The rays intersecting the faces of the light pipe nearest htlight coupling element such as the ray intersecting the point 1046 arecoupled in the waveguide device by the light coupling element. In theembodiment of FIG. 4 the light couple is a fold grating. The guide lightpath through the fold grating is indicated by the rays 1048. The rays1048 are then coupled in to the optical waveguide at the point 1047 toprovide the guided light path 1049.

In one embodiment the light coupling element may be a beam splitter asshown in FIG. 5 which illustrates a detail of the apparatus comprisingcross section of a light pipe 141, a light coupling elements 143comprising a beam splitter layer and an optical waveguide 142. In oneembodiment the beamsplitter is a thin metal coating with transmittancevarying along the length of the light pipe.

In one embodiment shown in FIG. 6 the apparatus 150 comprises an opticalsubstrate onto which is mounted a light pipe 152, a light couplingelement 152 and an optical waveguide 154 and a coupling grating 155. Thegrating comprises the grating elements 156. In one embodiment thecoupling grating has a non zero clock angle with respect to the opticalaxis. In other words, the projection of the coupling grating k-vector1071 in the plane of the substrate makes an angle with the light pipeoptical axis 1070. The effect of the clock angle is to produce thespiral-like TIR path in the light pipe. This principle may be applied inall of the embodiments of the inventions. A similar effect may beproduced by using a prismatic device. However, a clocked grating is themost elegant solution in terms of form factor. Typically, the clockangle will be around 45 degrees. Other angles may be used depending onthe light pipe geometry and angular constraints imposed by the lightcoupling element. Advantageously, the optical waveguide contains agrating having a reciprocal diffractive relationship with the light pipecoupling grating.

The embodiments of FIGS may be used to provide the first axis ofexpansion in a two axis beams expansion waveguide display as disclosedin The two axis expansion essentially provides a large exit pupil or eyebox. Using the present invention, the light pipe would provide the firstaxis of expansion and the optical waveguide the second (orthogonal) axisof expansion. Where a fold grating is used as the light coupling elementcare must be taken to make the spiraling light in the light pipeon-Bragg with the fold grating for particular ray vectors directionincident on the light pipe surface in optical contact with the lightcoupling element (that is, the fold grating). This is achieved byclocking the coupling grating at 45 degrees. Advantageously two couplinggratings are used for image injection into the light pipe in orderachieve pupil coverage of all field angles at the fold grating. The foldgrating then redirects the diffracted component the spiraling light intoa TIR path in the optical waveguide.

In most applications of the invention the optical waveguide will containa grating. Grating may be used to couple light out of the waveguide asdiscussed above. Such embodiments could be applied in waveguide displayssuch as the ones disclosed in U.S. Patent application Ser. No.13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY. In such embodimentthe coupling ling grating couples light modulated withtemporally-varying angularly-distributed information content into thelight pipe. The coupling grating may be used depending on theapplication. In another embodiment the grating will be used or toprovide in coupling from external source. This principle may be appliedin waveguide eye trackers such as the ones disclosed inPCT/GB2013/000210 entitled APPARATUS FOR EYE TRACKING.

In one embodiment the light coupling element is a grating comprising atleast two multiplexed gratings. Each grating may operate over a definedangular or spectral range. Multiplexing allows the angular bandwidth andcolor space to be expanded without significantly increasing the numberof waveguide layers. In one embodiment the light coupling element is agrating having at least one of the characteristics of spatially varyingthickness, spatially-varying diffraction efficiency, orspatially-varying k-vector directions. In one embodiment the grating hasa spatially varying thickness. Since diffraction efficiency isproportional to the grating thickness while angular bandwidth isinversely propagation to grating thickness allowing the uniformity ofthe diffracted light to be controlled. In one embodiment the grating hasspatially-varying k-vector directions for controlling the efficiency,uniformity and angular range of the grating. In one embodiment gratinghas spatially-varying diffraction efficiency. The application ofmultiplexing, and spatial varying thickness, k-vector directions anddiffraction efficiency in the present invention may be based on theembodiments, drawings and teachings provided in U.S. patent applicationSer. No. 13/506,389 entitled COMPACT EDGE ILLUMINATED DIFFRACTIVEDISPLAY, U.S. Pat. No. 8,233,204 entitled OPTICAL DISPLAYS, PCTApplication No.: US2006/043938, entitled METHOD AND APPARATUS FORPROVIDING A TRANSPARENT DISPLAY, PCT Application No.: GB2012/000677entitled WEARABLE DATA DISPLAY, U.S. patent application Ser. No.13/317,468 entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY, U.S.patent application Ser. No. 13/869,866 entitled HOLOGRAPHIC WIDE ANGLEDISPLAY, and U.S. patent application Ser. No. 13/844,456 entitledTRANSPARENT WAVEGUIDE DISPLAY.

In one embodiment shown in FIG. 7 the light pipe comprises a hollowcavity 160 with a mirror coating applied the internal surfaces. Thelight pipe which has a square cross section is shown in side view inFIG. 7A and in cross section 161 in FIG. 7B. In one embodiment shown inFIG. 8 the light pipe is a refractive element 180 fabricated from agradient index material. The light pipe is shown in side view in FIG. 9.The transverse refractive index distribution is shown in FIG. 9B.

In one embodiment shown in FIG. 9 the light pipe 190 is divided into twoelongate portions 191,192 by a beamsplitter layer 193. In one embodimentthe beamsplitter is a thin film coating. The light pipe is shown in sideview in FIG. 9A and in cross-section in FIG. 9B. In one embodiment thebeamsplitter provides 50/50 beams division. In another embodiment thebeamsplitter may be polarization selective.

FIGS. 10-13 show schematic side elevation views of a light pipe andlight coupling element combinations for use in the invention. In eachcase a short section 201 of a light pipe based on the square crosssection elements discussed above, with a portion of the spiral lightpath 1090 and a ray interaction 1091 with the light coupling element isillustrated. In the embodiment of FIG. 10 the light coupling element isa grating 202. In the embodiment of FIG. 11 the light coupling elementis a beamsplitter 203. In the embodiment of FIG. 12 the light couplingelement is a surface relief grating 204. In the embodiment of FIG. 13the light coupling element is a switchable grating array 205 containingswitchable elements such as 206. In the embodiment of FIG. 10 the lightcoupling element is a gradient index material 207 providing curved raypaths 1092. In each case the light coupling elements provides lightpaths into the optical wave guide that are at angles design to providehigh efficiency coupling with the waveguide grating. In other words theray angles fall within the diffraction efficiency angular bandwidth ofthe waveguide grating (which condition is often referred to as beingon-Bragg).

The embodiments of FIG. 1-6 may be used to transmit image light from amicrodisplay to the eyebox of a display. In the case of waveguidedisplays the input light is modulated with temporally-varyingangularly-distributed information content using a spatial lightmodulator such as a liquid crystal display panel or using a laserscanner based on MEMS or other beam deflection technology. Embodimentssimilar to those of FIG. 1-6 may be used to provide an illuminator. Theembodiments of FIG. 1-6, with rays reversed, may be used to illustrate afurther embodiment in which the optical waveguide grating is operativeto extract light propagating therethrough out of the optical waveguideor a grating operative to couple-in light from outside the opticalwaveguide. Such an embodiment may be used in a sensor such as an eyetracker or LIDAR system. The grating coupler would become an outputcoupler for directing signal light onto a detector. The benefit of thepresent invention is that the range of detection handles can be expandedto address the full angular capability of a waveguide. With regard toeye tracking the invention may be used in the waveguide eye trackersdisclosed in PCT/GB2014/000197 entitled HOLOGRAPHIC WAVEGUIDE EYETRACKER, U.S. Provisional Patent Application No. 62/071,534 entitledHOLOGRAPHIC WAVEGUIDE FOR TRACKING AN OBJECT IN 3D SPACE,PCT/GB2013/000210 entitled APPARATUS FOR EYE TRACKING, PCT ApplicationNo.: GB2013/000210 entitled APPARATUS FOR EYE TRACKING.

It should be emphasized that the drawings are exemplary and that thedimensions have been exaggerated. For example, thicknesses of the SBGlayers have been greatly exaggerated. Optical devices based on any ofthe above-described embodiments may be implemented using plasticsubstrates using the materials and processes disclosed in PCTApplication No.: PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHICPOLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES.

It should be understood by those skilled in the art that while thepresent invention has been described with reference to exemplaryembodiments, it is to be understood that the invention is not limited tothe disclosed exemplary embodiments. Various modifications,combinations, sub-combinations and alterations may occur depending ondesign requirements and other factors insofar as they are within thescope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A waveguide apparatus comprising in combination:a light pipe having a reflecting surface disposed around an optical axisfor guiding light therethrough; a light coupling element in opticalcontact with an elongate portion of said reflecting surface; and anoptical waveguide in optical contact with said light coupling element.2. The apparatus of claim 1 wherein said light is transmitted from saidlight pipe to said optical waveguide via said light coupling element. 3.The apparatus of claim 1 wherein said light is transmitted from saidoptical waveguide to said light pipe via said light coupling element. 4.The apparatus of claim 1 wherein said reflecting surface comprisesabutting mutually inclined elongate elements, at least one said elongateelement in optical contact with said light coupling element.
 5. Theapparatus of claim 1 wherein said reflecting surface comprises abuttingelongate elements mutually inclined at a common angle, at least one saidelongate element in optical contact with said light coupling element. 6.The apparatus of claim 1 wherein said light coupling element is one of agrating, a beam splitter, an evanescent coupling optical medium, or agradient index optical medium.
 7. The apparatus of claim 1 wherein saidlight coupling element is a fold grating.
 8. The apparatus of claim 1wherein said light coupling element is a surface relief grating.
 9. Theapparatus of claim 1 wherein said light coupling element couples lightcharacterized by at least one of angular range, wavelength range orpolarization state.
 10. The apparatus of claim 1 wherein said couplingelement is a Bragg grating, a switchable Bragg grating or an array ofselectively switchable elements and is recorded in one of a HPDLCgrating, uniform modulation grating or reverse mode HPDLC grating. 11.The apparatus of claim 1 wherein said coupling element is a gratingcomprising at least two multiplexed gratings.
 12. The apparatus of claim1 wherein said coupling element is a grating having at least one of thecharacteristics of spatially varying thickness, spatially-varyingdiffraction efficiency, or spatially-varying k-vector directions. 13.The apparatus of claim 1 wherein the optical medium of said light pipeis at least one of air, optical refractive material or a gradient indexmaterial.
 14. The apparatus of claim 1 wherein said optical waveguidecontains a grating operative to extract light propagating therethroughout of the optical waveguide or a grating operative to couple-in lightfrom outside the optical waveguide.
 15. The apparatus of claim 1 whereinsaid light guide device further comprises a coupling grating in opticalcontact with said light pipe, said coupling grating having a non zeroclock angle with respect to said optical axis.
 16. The apparatus ofclaim 1 wherein said light guide device further comprises a light pipecoupling grating in optical contact with said light pipe, wherein saidoptical waveguide contains a grating having a reciprocal diffractiverelationship with said light pipe coupling grating.
 17. The apparatus ofclaim 1 wherein said light guide device further comprises a light pipecoupling grating in optical contact with said light pipe, wherein saidlight pipe coupling grating couples light modulated withtemporally-varying angularly-distributed information content into saidlight pipe.
 18. The apparatus of claim 1 wherein said light guide devicefurther comprises a light pipe coupling grating in optical contact withsaid light pipe, wherein said coupling grating couples data modulatedlight out of said light pipe.
 19. The apparatus of claim 1 wherein saidlight pipe is divided into two elongate portions by a beamsplitterlayer.
 20. The apparatus of claim 1 wherein said light pipe is curved.